1. Field of the Invention
This invention relates to the prevention of reverse engineering of integrated circuits (ICs), and more particularly to security techniques in which interconnections between circuit elements are made undetectable.
2. Description of the Related Art
Several techniques have been used to reverse engineer ICs. Electron (e)-beam probing with a scanning electron microscope (SEM), either through SEM photographs or voltage contrast analysis, is the standard reverse engineering mechanism, although secondary ion mass spectrometry (SIMS), spreading resistance analysis and various other techniques have also been used. A general description of e-beam probing is provided in Lee, "Engineering a Device for Electron-beam Probing", IEEE Design & Test of Computers, 1989, pages 36-49.
Numerous ways to frustrate unwanted attempts to reverse engineer an IC have also been developed. For example, in U.S. Pat. No. 4,766,516 to Ozdemir et al. (assigned to Hughes Aircraft Company, the assignee of the present invention), additional circuit elements that do not contribute toward the desired circuit function are added to an IC, and disguised with the visible appearance of being an ordinary part of the IC. The elements have physical modifications that are not readily visible but cause them to function in a different manner, inhibiting the proper functioning of the IC in case of an attempted copying or other unauthorized use. When the apparent function rather than the actual function of the disguised elements are copied, the resulting circuit will not operate properly.
In U.S. Pat. No. 4,583,011 to Pechar a pseudo-MOS (metal oxide semiconductor) device is given a depletion implant that is not readily visible to a copier, who would infer from the device's location in the circuit that it would be enhancement-mode. A somewhat related approach is taken in French patent publication no. 2 486 717 by Bassett et al., published Jan. 15, 1982; the circuit doping is controlled so that some devices which appear to be transistors actually function as either open or short circuits. And in U.S. Pat. No. 4,603,381 to Guttag the memory of a central processing unit is programmed by the doping of its channel regions, rather than by the presence or absence of gates, to protect permanently programmed software.
Instead of disguising circuit elements, some systems have a mechanism to protect the circuit from operating until a correct access code has been entered. Such systems are described in U.S. Pat. No. 4,139,864 to Schulman and U.S. Pat. No. 4,267,578 to Vetter.
Each of the above protection schemes requires additional processing and/or uses additional circuitry that is dedicated to security and does not contribute to the basic functioning of the circuit. This increases the cost of circuit production and complicates the circuitry.